TABLE OF CONTENTS The future prospects of high-speed long distance optical fibre communication systems depend heavily on the availability of low-cost optical amplifiers which can compensate for the build up of losses in optical fibre cables over long distances. Two types of optical amplifier exist: (i) semiconductor laser diode amplifiers and (ii) fibre amplifiers. Semiconductor laser diode amplifiers are essentially laser diodes operating in the linear amplification region below oscillation threshold, whereas fibre amplifiers are optical fibres doped with Erbium ions (Er 3+) to provide optical gain. Semiconductor laser diode amplifiers have the inherent advantages of compactness and the possibility of integration with other optoelectronic components, whereas fibre amplifiers have the advantages of easy and efficient coupling to optical fibres. The design and analysis of both types of optical amplifier are therefore crucial for the future development of coherent optical communications.
To understand fully how optical amplification can be achieved, the interaction of electromagnetic radiation with matter must first be understood. Therefore, in this chapter, we will first explore the interaction of radiation with a simple two-level atomic system. This simple model provides the basis for studies of more complex quantum mechanical systems, including those of semiconductors [ [1]].
An understanding of the interaction of radiation with a two-level system enables one to understand the operation of optical amplifiers and from this their fundamental performance characteristics can be derived. In general, such characteristics can be used to describe both fibre amplifiers and semiconductor laser amplifiers [ [2]], so...
